This invention relates generally to the growth of semiconductor materials, and more particularly to the growth of thin semiconductor layers for use in heterostructure devices.
As it is known in the art, a heterostructure device is one in which semiconductor layers comprised of different materials having different electrical properties are disposed over one another providing a heterojunction. Examples of such devices are heterojunction field effect transistors (FETs), heterojunction bipolar transistors (HBTs), and optoelectronic devices. Selectively doped heterostructure devices can have higher electron mobility characteristics than, devices formed without heterojunctions due to the physical separation of free carriers from the charge donating atoms.
As it is also known in the art, heterojunction field effect transistors have important digital and analog, particularly microwave, applications. An example of such a heterojunction FET is the high electron mobility transistor (HEMT) which provides inter alia higher noise immunity characteristics than conventional digital FETs. Examples of gallium arsenide (GaAs)/gallium aluminum arsenide (GaAlAs) HEMTs grown by the technique of metalorganic chemical vapor deposition (MOCVD) are mentioned in an article by Ohori et al. in the Journal of Crystal Growth, Volume 93, pps. 905-910, 1988 where the interface between an InGaP layer and a GaAs layer provides the heterojunction of the HEMT device. With the device reported in the previous article, the sheet carrier concentration of the high electron mobility active layer of these structures is limited to 1.0.times.10.sup.12 carriers/cm.sup.2. This relatively low carrier concentration is undesirable because it reduces the current capability of the device.
As it is further known in the art, high electron mobility transistors having a channel layer comprised of gallium indium arsenide (GaInAs) exhibit improved performance characteristics over GaAlAs/GaAs HEMTs, and other HEMTs as mentioned above. More specifically the GaInAs/GaAs device is considered a pseudomorphic device since the adjacent layers are mechanically strained due to differences in the lattice constant of such layers. Thus, the HEMT device provided with such layers may be referred to as a pseudomorphic HEMT, or PHEMT. As described by J. J. Rosenberg et al. in IEEE Electron Device Letters, Volume EDL-6, No. 10, pps. 491-493, 1985, and W. T. Masselink et al. in Electronics Letters, Volume 21, #20, pps. 937-939, 1985, a pseudomorphic structure in which a thin GaInAs layer provides the channel layer results in higher DC and microwave performance capabilities than conventional HEMTs. The presence of the GaInAs channel layer provides a device having increased electron mobility, higher saturated velocity in the GaInAs channel layer, and larger conduction band discontinuity between the doping layer and the channel layer. Such improvements permit higher frequency operation and higher power handling capability.
One technique known in the art for providing GaInAs PHEMTs is molecular beam epitaxy (MBE). Molecular beam epitaxy is a growth technique in which source materials are placed in a vacuum chamber. Under such high vacuum conditions, the source materials evaporate and deposit onto a suitable substrate. While typically providing excellent material uniformity and control of abruptness of interfaces between epitaxial layers, MBE inherently has a relatively low throughput due to low deposition rate and limited deposition surface area and also requires a relatively complex and expensive reactor system.
As it is also in the art, metalorganic chemical vapor deposition (MOCVD) is a depositing process by which vapors of an organic compound containing an element or substance to be deposited are directed into a reactor and thermally decomposed to liberate the element or substance to be deposited. Such decomposition thus provides non-volatile reaction products which are deposited on a suitable substrate. This process can be scaled up to provide large deposition surface areas and, with a moderate deposition rate, yields high throughput. MOCVD may be carried out either under atmospheric pressure conditions or at a reduced pressure. The technique of MOCVD is sometimes referred to as metalorganic vapor phase epitaxy (MOVPE).
Reduced pressure MOCVD growth typically yields better material uniformity than such growth at atmospheric pressure. However, under such reduced pressure conditions, certain materials, in particular those having a relatively high vapor pressure at the growth temperature, will have increased evaporation of the elements to be deposited from the surface of the substrate. Such increased evaporation may inhibit film deposition or result in poor compositional uniformity. Also, reduced pressure MOCVD growth requires relatively complex apparatus in order to provide lower than atmospheric pressure conditions. This apparatus complexity will increase the overall process cost.
One approach suggested for providing GaInAs/InP PIN diodes by MOCVD uses an inverted growth cell and is described by N. Puetz et al. in the Journal of Electronic Materials, Volume 17, No. 5, pps. 381-386, 1988. There is no indication that the material provided by the authors would be suitable for PHEMT devices. Moreover, use of an inverted reactor complicates substrate handling and reactor manufacture and would be undesirable in a production environment.
The technique of MOCVD has been used to provide GaAlAs/GaAs and InGaP/GaAs HEMT devices, GaInAs/InP PHEMT devices, as well as GaInAs/InP PIN diode devices as described in the above referenced articles. Heretofore, the technique of MOCVD has not been generally used in providing pseudomorphic PHEMT devices having a GaInAs channel layer, because of the relative difficulty in providing compositionally uniform GaInAs layers across large areas as well as in providing such layers with abrupt junctions.
Compositional uniformity of a GaInAs layer grown by MOCVD is generally difficult to provide over large surface areas because of the relatively unstable characteristics of organic compounds used to provide a source of indium, particularly trimethylindium. In particular, at high pressures (i.e. atmospheric) gas phase depletion of the indium metal-organic occurs, whereas at low pressures increased evaporation of indium from the surface of the substrate occurs. Each of these conditions will result in a non-uniform distribution of indium in the GaInAs layer.
Moreover, GaAs, GaAlAs, and GaInAs layers are conventionally grown at different temperatures to accommodate different rates of gas phase depletion of the precursor compounds and to optimize material layer quality. In particular, GaAs layers are grown at approximately 650.degree. C., whereas layers containing indium are grown at lower temperatures and layers containing aluminum are grown at higher temperatures. While this permits fabrication of PHEMT devices, due to time constants involved in ramping to higher or lower temperatures, interdiffusion or alloying can occur between adjacent layers. Interdiffusion between adjacent layers is undesirable, particularly for the growth of thin film and ultra thin film heterostructure devices.
It would therefore be desirable to grow GaInAs PHEMTs, and other heterojunction devices having uniform material and thickness composition over relatively large area substrates for integrated circuit fabrication using the technique of MOCVD. Moreover, it would also be desirable to provide layers with abrupt heterojunctions. Such growth may be conducted at either atmospheric pressure or reduced pressure conditions. In general, MOCVD growth would offer the benefits of requiring less complex and less costly apparatus than the technique of MBE, and higher throughput. If the uniformity and transport properties of GaInAs PHEMTs previously obtained by using the technique of MBE can be provided by MOCVD, then these benefits would have been acquired with no detrimental effect on device characteristics.